1. Field of the Invention
This invention relates to lasers, and more particularly to improved multiple-color lasers.
2. Description of Related Art
Multiple-wavelength laser systems are used for a variety of applications. In the context of this invention, multiple wavelengths may be taken to mean two or more wavelengths that can be distinguished from each other and be used to convey independent information to the observer or detection apparatus.
One multi-wavelength application is full-color holograms. These holograms present holographic, full-color images. For obvious reasons, such holograms are much preferred to the older generation of monochromatic holograms. Color holograms may be recorded on holographic panchromatic materials. Ultra-high resolution, single-layer, silver-halide emulsions and new photo polymer materials also may be used for this purpose.
A variety of recording setups might be used for full-color holography. However, it appears that the single beam Denisyuk recording scheme has produced the best results with the simplest apparatus. Three laser wavelengths, such as the colors red, green, and blue, are needed for the recording. Suitable colors can be selected from different lasers conventionally in use in holographic recordings: argon, krypton, diode-pumped frequency-doubled Nd:YAG, helium-neon, and helium-cadmium lasers.
Multiple lasers are needed in this arrangement. A prior art color hologram recording set up is illustrated in FIG. 1. FIG. 1 shows color holography laser system 100, including HeNe laser 102; krypton laser 104; argon laser 106; first beam mirrors 108, 109, and 110; second beam mirror 112, and dichroic beam mirrors 114, and 116; optional coherence monitoring system 118; shutter 120, spatial filter 122, hologram recording film 124, and object 130.
HeNe laser 102, krypton laser 104, and argon laser 106 are installed on an independent vibration-isolation system isolated from an optical table surface (not shown). The beams emitted by the lasers are redirected to shutter 120 using first beam mirrors 108, 109, and 110; second beam mirror 112, and dichroic beam mirrors 114, and 116. An optional coherence monitoring system may be placed on the beam path between dichroic beam mirror 116 and the shutter. When the shutter is in the open position, a beam passes through spatial filter 122 and illuminates hologram recording film 124 and object 130. The object is positioned on a side of the hologram recording film opposite from the spatial filter.
In operation, the three colors of light emitted by HeNe laser 102, krypton laser 104, and argon laser 106 are combined into a single "white light" beam using first beam mirrors 108, 109, and 110; second beam mirror 112, and dichroic beam mirrors 114, and 116. Optional coherence monitoring system 118 may be used to monitor beam wavelength purity. The beam is then directed onto shutter 120, which serves to control illumination. When the shutter is open, the beam passes through spatial filter 122, and illuminates hologram recording film 124 and object 130. The light rays reflected from object 130 interfere with the beam incident on the hologram recording film to form a hologram, which is recorded by the hologram recording film.
Such an arrangement represented a significant improvement over earlier full-color hologram methods. Using the dichroic filters in combining laser beams permitted a shortened and simplified exposure procedure without changing mirror positions between exposures as was necessary before using dichroic mirrors. Furthermore, the light intensity and red-green-blue ratio on the recording plane were much less likely to remain undisturbed after initial set-up. This reduced the need for check-up and calibration between hologram recordings.
However, problems still remain with this arrangement. The use of multiple lasers and multiple optical elements significantly increases the cost of the recording system, making it commercially less feasible to produce inexpensive, custom holograms. Additionally, the relatively large number of laser systems and optical elements increases the possibility of failures and beam misalignments.
These problems result primarily from the fact that while conventional laser systems may emit multiple wavelengths, the desired colors are not available from any single laser gain medium. Further, the ratios of the available power at the various desired colors do not necessarily match those needed for conventional films. Additionally, the separation between the wavelengths is such that the emitted colors are not suitable for use in full-color holography. Therefore, the conventional solution, as illustrated in FIG. 1, has been to use multiple lasers with the attendant problems noted above.
Likewise, in other multiple-wavelength applications, such as three-wavelength laser Doppler velocimetry, conventional systems suffer from a number of shortcomings. Three-wavelength laser Doppler velocimetry systems using conventional lasers would require multiple lasers, with attendant cost and reliability issues.
FIGS. 2A, 2B, and 2C show a prior art three-dimensional laser Doppler velocimetry system. Shown in FIG. 2A are lasers 202, 204, 206, and optical fiber network 208. Shown in FIG. 2B are first focusing optic 210, X-direction beam 212, Y-direction beam 214, second focusing optic 216, Z-direction beam 218, measurement volume 220, and fiber optic network 208. Shown in FIG. 2C are second focusing optic 216, Z-direction beam 218, measurement volume 220, back scattered beam 222, detector 224, and signal path 226 to a signal analyzer (not shown).
Lasers 202, 204, and 206 are optically coupled to optical fiber network 208. Optical fiber network 208 is optically coupled to first focusing optic 210, and to second focusing optic 216. Detector 224 is positioned in the optical path behind focusing optic 216 in such a way as to capture back scattered light from measurement volume 220. Detector 224 is coupled by signal path 226 to a signal analyzer (not shown).
In operation, lasers 202, 204, and 206 emit light into fiber optic network 208. Each of lasers 202, 204, and 206 emit on a single wavelength or a single color. Light transmitted by the fiber optic network is delivered to first focusing optic 210, and second focusing optic 216. First focusing optic 210 serves to focus X-direction beam 212 and Y-direction beam 214 that are used to measure velocities in measurement volume 220 in both the X and Y directions. The Z direction measurement is made by light transmitted by fiber optic network 208 focused through second focusing optic 216 and directed as Z-direction beam 218 to measurement volume 220. Results from the measurement volume are captured via back scattered beams, for example, back scattered beam 222, shown in FIG. 2C. The back scattered beam is collected through second focusing optic 216 onto detector 224. The signal from the detector is then transmitted to an analyzer via signal path 226. Although FIG. 2C shows an arrangement for the detection for measurements in the Z direction, similar detector arrangements may be used to monitor the X and Y directions as well.
A problem with current laser Doppler velocimetry systems is that they use laser sources with relatively broad linewidths. These linewidths are typically on the order of 6-8 Giga-hertz measured full width at half maximum. Such relatively broad linewidths translate to relatively short spatial coherence lengths which limit the size of the measurement volume as defined by the interference fringes at the intersection of the two beams. In addition, the relatively broad linewidths also limit the contrast of the interference fringes and thus the signal-to-noise ratio of the laser Doppler velocimetry measurement.
There is therefore a need for a single laser system that addresses these deficiencies in the prior art.